OSB is made from flakes that are created from debarked round logs by placing the edge of a cutting knife parallel to a length of the log and the slicing thin flakes from the log. The thickness of the flake is approximately 0.010 to 0.030 inch. The cut flakes are subjected to forces that break the flakes into strands having a length parallel to the grain of the wood several times the width of the strand. The strands can be oriented on the board forming machine with the strands predominantly oriented in a single, e.g., cross-machine direction in one, e.g., core layer and predominantly oriented in the generally perpendicular (machine) direction in adjacent layers. The various layers are bonded together by natural or synthetic resin(s) under heat and pressure to make the finished OSB product.
The common grade of OSB is used for sheathing walls and decking roofs and floors where strength, light weight, ease of nailing, and dimensional stability under varying moisture conditions are the most important attributes. In these applications, the appearance and/or weathering of the rough surfaces are not of concern since the product will be covered with roofing, siding, or flooring. Because of the unfinished attributes of utility grade OSB, it commands a relatively low price in the marketplace and is sold at a discount to structural grades of softwood plywood.
The light weight, ease of nailing, and dimensional stability of OSB are attributes much desired in siding products but, due to the irregular surface, OSB has required surface modification before being used as siding or otherwise where aesthetics is important to the consumer. If the material could be imparted with the surface smoothness, coatability, and weatherability of hardboard while retaining its other desirable structural properties, it would be significantly improved in comparison to the commodity structural grade. Others have pursued this objective along different lines with partial success.
One attempt to prevent "telegraphing" is described in Greten U.S. Pat. No. 3,098,781. The Greten '781 patent discloses a particleboard product made from materials, such as flakes, wherein the flakes are graduated in size from the center or core to the outer surfaces, with the coarsest flakes at the core and the finer flakes, together with fines, at one or both outer surfaces. The Greten produced particleboard is disclosed to have the advantage of accepting an overlay of veneer, paper or plastic sheets without "telegraphing" the relatively irregular surface of the underlying particleboard.
Similar OSB siding products are commercially sold that include a resin-bonded overlay of paper laminated to one surface. The paper can accept a limited degree of embossing but it cannot stretch to accept deep embossing. When embossed beyond a certain depth, the paper ruptures from the tensile strain and reveals the underlying flakes. Furthermore, exposure to the weather causes irreversible swelling of the flakes in thickness which telegraphs the structure of the underlying baseboard (OSB) through the thin overlay and creates a bumpy, irregular exposed surface. The result is an unsightly appearance of the front surface, especially of product that is unembossed or only slightly embossed.
Another example is described in Wentworth U.S. Pat. No. 4,364,984 where wood fines are distributed on the surface of the flake baseboard (OSB) graduated with the coarsest wood fines adjacent to the flakes and the finest on the visible surface. Since the fines are bundles of wood fibers which retain the stiffness of wood, they do not consolidate into a tight surface, but rather, retain susceptibility to the ready entry of water and do not holdout paint to a satisfactory degree.
Similarly, Ufermann, et al. U.S. Pat. No. 4,068,991 discloses a particleboard, e.g., chipboard product that includes a continuous particle size gradient between a coarser particle core and a finer particle surface layer wherein the particle size gradient transition from one particle size to another can be continuous or step-wise.
Others have disclosed the manufacture of laminates of plywood or particleboard with a wet-process fiberboard surface, e.g., Birmingham U.S. Pat. No. 2,343,740; Bryant 3,308,013 and Shaner, et al. U.S. Pat. No. 4,361,612 discloses forming an oriented strand board (OSB), that may be in three or more layers, formed from a mixture of hardwood species and then laminating the OSB to a veneer, wet-process hardboard or plywood face panel.
One of the problems associated with the application of an overlay onto an OSB baseboard is that of achieving a strong bond at the interface between the OSB and the overlay capable of resisting weathering. The above-described Wentworth U.S. Pat. No. 4,364,984 suggests that a strong bond can be achieved at the interface between an OSB product and a fine particle overlay by manufacturing the OSB with the largest OSB flakes at the interface, and applying the overlay fine particles such that the longest fines are disposed at the interface. Similarly, the Shaner, et al. U.S. Pat. No. 4,361,612 discloses that shorter fibers in the surface of an OSB product will degrade the bending strength of an OSB product. Further, the Shaner '612 patent teaches that a laminated wood product including a flakeboard core laminated to a wood veneer, a wet-process hardboard or a wet-process fiberboard overlay, as in typical plywood practice, may need a core finishing operation on a drum sander to achieve a core surface capable of good bonding to the overlay.
Bryant U.S. Pat. No. 3,308,013 suggests that a water-laid fiber sheet containing resin and having a basis weight of dry fiber from 30 to 750 pounds per thousand square feet can be employed to mask defects in plywood, particleboard, and the like. These heavy papers have been used to produce medium density overlain plywood that has found application in road signs where the smooth surface accepts lettering and reflective laminates. High cost, limited embossability, poor weathering, and poor adhesion of coatings preclude the use of this product in siding applications.
It has heretofore been generally accepted by those skilled in the art that an OSB baseboard and a fiberboard overlay will not form a good bond at their interface and that the differential in dimensional and elastic properties of the fiberboard and OSB materials will result in delamination because of moisture cycling due to weather conditions. This conventional wisdom also advised against using dried board trim waste as a raw feed to the fiber pulping operation because of residual bonded and consolidated resin. While this theory has been verified for OSB wet-process fiberboard composite structures, surprisingly and unexpectedly, excellent bonding and resistance to weathering is achieved in accordance with one embodiment of the present invention by applying a fiberboard overlay by the dry process to an OSB baseboard. Additional advantages are achieved in the preferred embodiment by forming the OSB such that the smallest flakes of the OSB are disposed at the fiberboard interface, as will be described in more detail hereinafter.
In the prior art manufacture of OSB, a warping problem was encountered when the OSB was formed from three OSB layers using a screen within the platen press for final consolidation of the three strand layers into a unitary OSB structure. It was theorized that the screen marks on the one OSB surface layer increased the amount of effective surface area on that OSB surface layer, thereby causing the warping problem. In order to compensate for warpage, it was found in the prior art that warping could be prevented by increasing the thickness of the screen-marked (higher surface area) OSB surface layer, in comparison to the thickness of the OSB surface layer without screen marks, an amount such that the screen-marked OSB surface layer had a thickness 15% higher than one-half the total thickness of the OSB surface layers surrounding the OSB core layer. Typically, warping was prevented in prior art OSB manufacture, wherein one of the surface layers of the OSB included screen indentations, by providing a three-layer OSB product such that the top (non screenmarked) OSB layer comprised 31.6%-33.4% of the total OSB thickness; the center OSB layer comprised 42%-45% of the total OSB thickness; and the lower (screen-indented) OSB layer comprised 23.4%-24.6% of the total OSB thickness. Thus, the top OSB layer (not screen-indented) having the smaller surface area was made 15% thicker than 1/2 the sum of top and bottom OSB layer thicknesses to prevent warping, with the central OSB layer, oriented perpendicularly to the machine direction, comprising 42%-45% of the total board thickness.
It has been found, in accordance with one embodiment of the present invention, that to achieve excellent embossing fidelity (the capability of achieving a sharp, accurate and permanent transference of an embossing plate design from an embossing plate to a board surface) in an OSB fiberboard overlay, the fiberboard overlay should be air-laid (formed by the dry process). If the fiberboard overlay applied over an OSB surface is water-laid (formed by the wet process), as suggested in the prior art, the sharp corners and other embossing precision necessary for high quality transference of an embossing plate design is not possible.
Unexpectedly, it has been found that the application of a dry layer of a mixture of defibrated fiber and resin binder over an OSB surface enables exact and precise transference of embossing plate details into the surface of the fiberboard overlay. Further, the bonding achieved at the interface between the OSB and the dry process fiberboard overlay, and the resistance to weathering of the fiberboard overlay are unexpectedly better when the fiberboard overlay is formed into a loose, but handleable mat formed by the dry process (the fibers are laid onto a support surface by gravity from a mixture with air, or mechanically, and are contacted with a binder resin during the fall of fibers onto the support surface, and generally contain less than about 15% water) and the fiberboard overlay and OSB layers are consolidated in a hot press simultaneously. As set forth in more detail hereinafter, the bonding is unexpectedly higher and the boil swell values substantially lower for the OSB-fiberboard composite products of the present invention than for a similar product that includes a fiberboard overlay applied by the typical wet process.
Furthermore, those skilled in the art have anticipated warping of the product if the overlay were applied only to one surface but, in accordance with another embodiment of the present invention, it has been found that the expected warping does not occur even in full size panels, e.g., 4'.times.8', when the fiberboard overlay is applied to only one major surface, and the thicknesses of the underlying OSB layers are carefully selected, as described in more detail to follow.
The multi-layer OSB-fiberboard composite structure of the present invention, having one fiberboard surface layer and the other surface layer formed from an OSB layer without screen indentations, and having at least two OSB layers therebetween, has substantially different characteristics and physical properties from an OSB without the overlay and, therefore, was completely different in terms of possible warp or cupping during manufacture.
Initial experimental trials in the manufacture of the OSB-fiberboard composite of the present invention on a commercial scale, having three OSB layers of equal thickness, and a surface layer of fiberboard over one of the outer OSB layers resulted in a board that cupped or warped substantially, even with light weight overlays, e.g., 150 pounds per thousand square feet, leading to the present invention.
In accordance with another embodiment of the present invention, warping or cupping of OSB-fiberboard composite structures can be eliminated with careful selection of OSB layer thicknesses, regardless of whether the fiberboard layer is applied by the wet or dry process, as described in more detail hereinafter.